OBD2 Diagnostic Code

P0117

Engine Coolant Temperature Circuit Low Input — What to Check First

Q: Can I drive with P0117 active?

Short distances only with caution. The ECM uses coolant temperature to control fuel enrichment, ignition timing, and fan operation. With P0117 active, the ECM typically defaults to a rich fuel strategy assuming a cold engine. This wastes fuel, increases emissions, and can foul spark plugs. It also disables some protection features against overheating. Do not ignore it for more than a day or two.

Q: Why does P0117 set on a warm engine?

A warm engine reading that still triggers P0117 almost always points to a wiring or connector fault rather than a failed sensor. At operating temperature, a healthy ECT sensor produces roughly 0.5V to 0.7V. If the ECM still sees below 0.2V after warm-up, the signal circuit has a short to ground or the connector is corroded. The sensor itself would need to be completely failed to produce that reading on a warm engine.

Q: What voltage should the ECT sensor read?

A healthy coolant temperature sensor uses a thermistor that changes resistance with temperature. Cold engine (around 32°F/0°C): approximately 3.5V to 4.5V. Normal operating temperature (around 194°F/90°C): approximately 0.5V to 0.8V. P0117 sets when the ECM sees voltage below approximately 0.2V, which corresponds to a temperature reading above 275°F (135°C) — physically impossible under normal conditions.

Q: Will P0117 cause my engine to overheat?

Not directly, but it can hide an overheating problem. If the ECT sensor is reporting a falsely high temperature, the ECM may not respond correctly to a real overheating event. More critically, the electric cooling fan on many vehicles is controlled by the ECT signal — a failed signal can prevent the fan from activating at the right time. Watch your temperature gauge independently and never rely solely on the check engine light when P0117 is active.

P0117 sounds scarier than it usually is. In most cases it's not a failed sensor — it's a corroded connector or a broken wire that costs next to nothing to fix. Before you buy a new coolant temp sensor, spend 10 minutes checking these three things first.

Severity

6 / 10

Moderate. ECM runs on a bad default strategy. Fix it within a few days.

Safe to Drive?

Briefly

Short trips to a shop are manageable. Do not ignore it — the ECM is flying blind on temperature.

Typical DIY Fix Cost

Very Low

Wiring and connector repairs often cost nothing. Sensor replacement is inexpensive on most vehicles.

🔍 What This Code Means

Your engine has a coolant temperature sensor — a small probe screwed into the engine block or coolant housing — that tells the ECM (the engine's computer) how hot the coolant is at any given moment. The ECM uses that reading to set fuel mixture, ignition timing, idle speed, and cooling fan activation.

The sensor works by changing its electrical resistance with temperature. Cold coolant produces high resistance, high voltage. Hot coolant produces low resistance, low voltage. P0117 sets when the ECM sees a voltage below approximately 0.2 volts on the sensor signal wire — a reading that would imply a coolant temperature above 275°F (135°C), which is physically impossible under normal operation.

💡 The ECM interprets P0117 as an implausibly hot engine. It usually responds by defaulting to a rich fuel strategy — dumping extra fuel as if the engine were cold — which wastes fuel and can foul spark plugs over time.

The low voltage reading is almost never caused by the engine actually running that hot. It means the signal circuit has a problem: either the sensor itself has failed short, there is a short to ground in the wiring, or the connector is corroded badly enough to drop the voltage artificially.

⚡ Common Symptoms

What you actually feel behind the wheel depends on how the ECM responds. Some vehicles enter a noticeable limp-home mode; others just throw the light with minimal drivability impact.

Check engine light on

Often the only symptom on newer vehicles with robust fallback strategies. The light is the code's most reliable calling card.

Poor fuel economy

ECM over-fuels constantly, believing the engine is always cold. Noticeable MPG drop, especially in city driving.

Rich-smell exhaust

Excess fuel that doesn't fully combust exits as unburned hydrocarbons. A strong gasoline odor from the tailpipe is a giveaway.

Hard warm start or rough idle after warm-up

The ECM keeps the mixture rich long after the engine has reached operating temperature, causing flooding-type symptoms.

Cooling fan runs constantly or doesn't run at all

The ECM controls electric fan speed using the ECT signal. A lost signal can result in either extreme — full on or full off.

Coolant temp gauge reads maximum or pegged high

On some vehicles the gauge and the ECM share the same sensor signal. A shorted circuit pins the needle at the top of the scale.

🎯 Most Likely Causes

Ranked by how often each cause actually turns out to be the culprit. Start at #1 — most P0117 cases are resolved before you ever touch the sensor.

Corroded or damaged ECT sensor connector The small 2-pin connector at the sensor is constantly exposed to heat cycles, coolant leaks, and road spray. Corrosion builds resistance in the ground pin and can pull signal voltage down below the P0117 threshold. This is the #1 cause — and it costs nothing to fix with a connector cleaning or a new pigtail.

~ 38% of cases

Short to ground in the ECT signal wire If the signal wire chafes against a metal bracket or ground point anywhere between the sensor and the ECM, it bleeds voltage to ground and registers as a low input. Common on vehicles where the harness runs near the exhaust or is routed through tight passages in the engine bay.

~ 27% of cases

Failed ECT sensor (shorted internally) A sensor that has failed short internally sends near-zero voltage regardless of actual coolant temperature. Most common after 100,000+ miles or following engine overheating events that cook the sensor element. The sensor itself is inexpensive on most vehicles.

~ 22% of cases

Low coolant level An ECT sensor that isn't fully submerged in coolant gives erratic or implausibly high temperature readings. Check the reservoir before anything else — it takes 30 seconds and has saved many people an unnecessary diagnostic bill.

~ 8% of cases

Faulty ECM (rare) In very rare cases the ECM's internal input circuit for the ECT signal develops a fault that reads low voltage regardless of what the sensor does. Only consider this after all wiring, connector, and sensor possibilities have been fully ruled out with a known-good test sensor.

~ 5% of cases

🔧 What to Check First

These three checks cover the causes behind roughly 70% of all P0117 cases. Do them in order. You don't need a scanner for any of them — just your eyes and a basic multimeter if you have one.

Check the coolant level first Pop the hood and look at the coolant overflow reservoir. The level should be between the MIN and MAX lines when cold. If it's low, top it up with the correct coolant type (check your owner's manual — mixing types causes problems), clear the code, and drive 15 minutes. A low coolant level exposes the sensor tip to air rather than liquid, producing exactly the kind of erratic voltage that triggers P0117. This takes 2 minutes and is often the entire fix.

Inspect and clean the ECT sensor connector With the engine cold, unplug the 2-pin connector at the coolant temperature sensor. Look carefully at both terminals — you're looking for green corrosion, white powder deposits, or bent/pushed-back pins. A corroded connector produces exactly the low-voltage reading P0117 describes. If you see any corrosion: spray the terminals with electrical contact cleaner, work a small wire brush or folded sandpaper through each socket, dry thoroughly, and apply a small amount of dielectric grease before reconnecting. Clear the code and test-drive 20 minutes.

Trace the ECT wiring harness for chafing or shorts Follow the sensor wire from the connector back toward the firewall. Look for spots where the harness rubs against a metal bracket, exhaust component, or sharp edge. A bare copper wire touching any grounded metal surface pulls the signal voltage to zero instantly. If you find a damaged section, wrap it tightly with quality electrical tape or heat-shrink tubing — or replace the affected section with a butt connector and new wire. Don't rely on a single strip of tape over a worn spot; the heat cycles will peel it off within weeks.

⚠️ If all three checks look clean, the sensor itself is the most likely culprit. A new ECT sensor costs very little at most auto parts stores and takes about 15 minutes to swap on most engines. Always let the engine cool fully before removing the sensor — coolant under pressure at operating temperature causes serious burns.

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⚠️ Common Misdiagnosis

P0117 has a straightforward list of causes — but there are three ways shops commonly get it wrong and send you home with an expensive repair that didn't solve anything.

⚠ Misdiagnosis #1 — Replacing the sensor before checking the connector

The single most common unnecessary repair for P0117. A corroded connector produces the exact same low-voltage signal as a failed sensor. The correct first step is always to unplug, inspect, clean, and retest. Any shop that quotes a new sensor without first demonstrating the connector is clean and making good contact is skipping the diagnostic process entirely.

Potential savings: Sensor cost + labor, when a free connector cleaning was the actual fix

⚠ Misdiagnosis #2 — Treating a wiring fault as a sensor fault

If the signal wire has a short to ground anywhere between the sensor and the ECM, the voltage at the sensor connector will read correctly — but the ECM will still see near-zero volts. Installing a new sensor doesn't fix a wiring short. The code will return within one drive cycle. Before replacing any sensor, check the voltage at the ECM harness connector, not just at the sensor end.

Potential savings: Avoiding a second sensor purchase after the first one doesn't fix it

⚠ Misdiagnosis #3 — Replacing the thermostat for P0117

A stuck-open thermostat causes P0128 (coolant temperature below thermostat regulating temperature), not P0117. These are completely different problems. P0117 is a circuit voltage issue — the sensor, wiring, or connector. A thermostat replacement will not touch the low-voltage fault. Don't let a shop bundle this repair in without a clear explanation of why it's needed for your specific situation.

Potential savings: Full thermostat replacement cost and labor

💰 Estimated Repair Cost

DIY cost = parts only. Shop cost = parts plus labor. Ranges vary by vehicle, region, and shop rate. Use these as a planning guide — not a fixed quote.

Connector cleaning (contact cleaner + dielectric grease)

DIYMinimal

ShopLow

LevelVery Easy

Connector pigtail replacement

DIYVery Low

ShopLow

LevelEasy

Wiring repair (short to ground)

DIYVery Low

ShopLow–Moderate

LevelEasy–Moderate

ECT sensor replacement

DIYLow

ShopLow–Moderate

LevelEasy

Coolant top-up

DIYMinimal

ShopVery Low

LevelVery Easy

ECM diagnosis and repair (rare)

DIYNot recommended

ShopHigh

LevelProfessional

✅ P0117 is one of the cheapest codes to fix when diagnosed correctly. The connector + wiring checks take under 20 minutes and cover the majority of cases. Only move to sensor or ECM replacement after those steps have been completed and failed.

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🚗 Most Reported On

P0117 can appear on any vehicle with an ECT sensor — which is every modern car and truck. These models report it most frequently based on owner records and verified repair data.

Chevrolet Silverado (2003–2009, 4.8L / 5.3L / 6.0L) GMC Sierra (2003–2009, 4.8L / 5.3L) Dodge Ram 1500 (2003–2008, 4.7L / 5.7L Hemi) Ford F-150 (2004–2008, 4.6L / 5.4L) Honda Accord (2003–2007, 2.4L / 3.0L) Toyota Camry (2002–2006, 2.4L) Jeep Grand Cherokee (2005–2010, 3.7L / 4.7L / 5.7L) Pontiac Grand Prix (2004–2008, 3.8L)

❓ Frequently Asked Questions

Can I drive with P0117 active?

Short distances to a shop are manageable with caution. The ECM uses the coolant temperature signal to set fuel mixture, ignition timing, and fan activation. With P0117 active, it typically defaults to a rich fuel strategy — assuming the engine is always cold — which wastes fuel, increases emissions, and can foul spark plugs over time. It also disables certain overheating protections. Don't ignore it for more than a day or two of regular driving.

Why does P0117 set on a warm engine?

A warm engine still triggering P0117 is a strong sign of a wiring or connector fault rather than a failed sensor. At normal operating temperature, a healthy ECT sensor produces roughly 0.5V to 0.7V. If the ECM still sees below 0.2V after the engine has fully warmed up, the signal circuit has a short to ground somewhere between the sensor and the ECM. The sensor itself would have to be completely failed internally to produce that reading on a warm engine.

What voltage should the ECT sensor read?

The ECT sensor is a thermistor — its resistance (and therefore voltage) changes with temperature. At a cold engine around 32°F (0°C), expect roughly 3.5V to 4.5V. At normal operating temperature around 194°F (90°C), expect roughly 0.5V to 0.8V. P0117 sets when the ECM sees voltage below approximately 0.2V — a level that would imply coolant hotter than 275°F (135°C), which is physically impossible under normal conditions.

Will P0117 cause my engine to overheat?

Not directly, but it can hide a real overheating event. The electric cooling fan on many vehicles is partially controlled by the ECT signal — a lost signal can prevent the fan from activating at the correct time. More importantly, if the sensor is reporting a false temperature, the ECM cannot respond correctly to actual overheating. Always monitor your temperature gauge independently and never rely solely on the check engine light when P0117 is active.

Is P0117 the same as P0118?

No — they are opposite faults. P0117 is a low voltage input (circuit reads below threshold), while P0118 is a high voltage input (circuit reads above threshold). P0117 usually indicates a short to ground in the signal wire or a sensor failed short internally. P0118 usually indicates an open circuit in the signal wire, a broken sensor, or a damaged connector that has pulled apart entirely. The diagnostic steps are different for each, so confirm which code your scanner shows before starting any repair.

🔗 Related Codes

These codes share similar root causes or commonly appear alongside P0117.

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Alex Carter ASE-Certified Technician · 14 years diagnostic experience

✓ Verified by OBDPlatform Team

This guide is for informational purposes only. Always consult a qualified mechanic before performing any vehicle repair. OBDPlatform is not responsible for actions taken based on this content.